Improved Armor Fortification

Improved Armor Fortification [Armor Focus]
You become less vulnerable to critical hits and sneak attacks in a chosen type of armor.
Prereqs: BAB +8, Armor Focus, Improved Armor Focus, Armor Fortification
Benefit: Choose a type of medium or heavy armor for which you've taken the Armor Focus, Improved Armor Focus, and Armor Fortification feats. When wearing this type of armor you gain the benefit of medium fortification (critical hits and sneak attacks have a 50% chance of being reduced to normal damage). You lose this benefit when denied your Dex bonus to AC.
Special: You may take this feat more than once. You must choose a different type of armor each time.
Notes: Moderate fortification has been reduced to 50% effectiveness in my campaign from 75%. This makes the math tidier (percentage doubles each time) and fixes what seems to me to be disproportionate gains for the ranks of fortification.